Many uses https://www.sketchapp.com/ but it's only available for Mac, not open source, and not free.
Another good one is https://www.figma.com/ which can be used online therefor works on any platform.
I prefer cross platform and open source tools so I found https://pencil.evolus.vn/ and I am curious what others are doing. Do some of you skip the prototyping phase completely? I assume if the team is small (1-3 devs), it's less needed.